Percutaneous vertebroplasty is a well-known procedure involving the injection of a bone cement or suitable biomaterial into a vertebral body via percutaneous route under X-ray guidance, typically lateral projection fluoroscopy. The cement is injected as a semi-liquid substance through a needle that has been passed into the vertebral body, generally along a transpedicular or posterolateral approach. The three main indications are benign osteoporotic fractures, malignant metastatic disease and benign tumours of the bone.
Percutaneous vertebroplasty is intended to provide structural reinforcement of a vertebral body through injection, by a minimally invasive percutaneous approach, of bone cement into the vertebral body. See, for example, Cotton A., et al "Percutaneous vertebroplasty: State of the Art." Radiograhics 1998 March-April; 18(2):311-20; discussion at 320-3. Percutaneous vertebroplasty can result in increased structural integrity, decreased micromotion at the fracture site, and possibly a destruction of pain fibres due to the heat of the bone cement as it polymerizes and sets. Complete pain relief can be achieved in up to eighty percent of patients. As known to those of skill in the art, the cement should have properties that, when injected, can increase vertebral body stiffness and compressive strength. It is generally preferred that the cement is liquid enough to flow into fracture planes and to fuse them. There is some debate about the appropriate thermal properties, but it is believed by some that the heating effect can be beneficial and cause death to local nerve endings involved in pain stimulation. It is generally accepted that most pain relief is achieved due to increased structural integrity.
The general steps for performing a vertebroplasty are as follows. The patient is placed in the prone position and the skin overlying the fractured vertebrae is prepped and draped. A suitable local anaesthetic such as 1% Lidocaine is injected into the skin underlying fat and into the periosteum of the pedicle to be entered. Next, a skin incision of about five millimetres is made with a No. 11 scalpel blade or other suitable surgical implement. The decision regarding which pedicle to use is made based on CT (computed tomography) and MR (magnetic resonance) images. A needle of an appropriate gauge (such as eleven gauge or thirteen gauge in a smaller vertebral body) is passed down the pedicle until it enters the vertebral body and reaches the junction of the anterior and middle thirds. This area is the region of maximum mechanical moment and usually the area of greatest compression. At this point a vertebrogram can be performed, if desired, by the injection of non-ionic X-ray contrast into the vertebral body to look for epidural draining veins.
Next, a cement is prepared. One suitable cement is a mixture of barium powder with methyl methacrylate powder and with a monomer liquid added to the mixture. Known cement products include Howmedica Simplex from Stryker Corporation 6300 Sprinkle Road, Kalamazoo, Mich. 49001, Osteobond from Zimmer Inc., 1800 West Center Street, Warsaw Ind. 46580, and Codman Cranioplastic from Codman, A Johnson & Johnson Company, 325 Paramount Drive, Raynham, Mass. 02767. From the moment that the monomer liquid is added to the powder there are generally about four to eleven minutes, with an average of about five to six minutes, before the cement thickens and becomes unworkable. Cement is inject under lateral X-Ray projection fluoroscopy imaging. The posterior aspect of the vertebral body is an important area to observe for posterior extension of cement, and it is generally accepted that this should be watched constantly during the injection. The injection is stopped as the cement starts to extend into some unwanted location such as the disc space or towards the posterior quarter of the vertebral body, where the risk of epidural venous filling and hence spinal cord compression is greatest. The injection is also discontinued if adequate vertebral filling is achieved. On average, about four to five cubic-centimetres of cement can be injected on each side, and it is known to inject up to about eight to nine cubic-centimetres per side.
About forty percent of the time it is possible to adequately fill a vertebral body from a single injection. It the vertebral body is not adequately filled during the first injection, it is necessary to pass a second needle down the other pedicle and inject more cement. The second side is anaesthetized, and a second skin incision is made. A second needle is passed down the other pedicle and advanced into the vertebral body to the junction of the anterior and middle third of the vertebral body. Again, cement is mixed and injected under lateral projection fluoroscopy imaging.
However, as is known to those of skill in the art, it can be very difficult to visualize the injection of cement against the existing cement from the first injection. Under the lateral projection fluoroscopy imaging, the second injection is visualized as a gradual subtle increase in the overall density or when the cement therefrom flows beyond the boundaries of the existing cement. In other words, the second injection cement is often not clearly identified until it enters an area, as viewed using lateral projection fluoroscopy, that the first injection did not fill. Due to the difficulty of observing the flow of cement during the second injection, the risk of spinal cord compression, unwanted venous filling and/or related damage, is increased. In particular, it is a challenge to observe when the cement extends posteriorly towards the epidural venous plexus, a group of veins that lie anterior to the spinal cord. Over injection can lead to the filling of these veins with cement and result in paraplegia or severe nerve compression. Pulmonary embolism is also possible.
Some of the foregoing difficulties may be overcome using a biplane angiography, where it is possible to see the cement in two planes simultaneously. However, it is not possible to judge the depth by the anterior-posterior projection and does not overcome the issue of control on the lateral projection of the posterior extent of the degree of cement filling. Furthermore, the expense and rarity of biplane angiographic equipment can make this an inaccessible option of many patients.